Embodiments of the invention relate to methods to produce micro fibrillated cellulosic material from pulp material.
Cellulose is an organic compound that makes up the structural component of the cell wall in plants and many forms of algae. It is also the most common organic compound on Earth, as well as Earth's most plentiful renewable resource. Micro fibrillated cellulose (“MFC”) comprises a series of micro fibrils that have been separated from their original cellulose fiber. MFC fibers are extremely fine, usually comprising of numerous cellulose chains. MFC typically has a width ranging from 5-20 nanometers and a length ranging from tens of nanometers up to several microns. MFC can be produced from any cellulose source; however, wood pulp is the most commonly used feed material in MFC production.
The increased surface area of MFC allows it to have a much higher number of hydrogen bonds binding the fibrils together, and because of this, MFC has uniquely high strength properties. Additionally, because of an increased ability to use abundant and renewable feed material to produce MFC, MFC has gained attention as a feedstock for material with multiple uses including packaging and composite reinforcement, and has shown potential to replace petroleum-derived polymers.
Many processes have been identified for producing MFC. These processes include cryocrushing, homogenization, microfluidization, and micro-grinding. In cryocrushing, the feed material is frozen using liquid nitrogen prior to high impact forces being applied to separate the fibrils from the cell wall.
In the homogenization process, the feed material undergoes rapid pressure decreases; typically the pressure drop is around 8,000 psi. Homogenization can be scaled for larger production and can be run continuously. Homogenization is a commonly used process for MFC production and one of the easier processes to scale up to larger production. This process can be very energy intensive, commonly requiring greater than 30 MWh/ton. In order to reduce the energy requirements for the homogenization process to produce MFC, carboxymethylation, TEMPO-mediation oxidation and many other chemical pre-treatment steps have been used, but these pre-treatment steps can be very expensive.
Microfluidizers compress the feed material and operate at a constant shear. Microfluidizers can be manufactured with differing geometries in order to produce materials with varying size.
Micro-grinding is similar to disc refining. During this process, feed material is pushed through a gap between a rotating and a stationary disc. These discs have grooves that contact and separate the fibers. The equipment used for the micro-grinding process may have rotor and stator disc surfaces coated with silicon carbide to assist in grinding.
A micro-grinding process to produce MFC has been discussed in EP 1538257 (“EP '257”) to Japan Absorbent Technology Institute. EP '257 presents a method to produce micro fibrillated cellulose using a disc refiner in recirculation beginning with pulp produced from cellulosic material. This patent describes a process for obtaining MFC produced from pulp derived from cellulosic material by mechanically refining the pulp with a disc refiner. The MFC produced from the mechanical refining of the pulp described in EP '257 have fibers with a length of less than 0.2 mm and a water retention value of 10 mL/g or greater. In this process, prior to the mechanical refining of the pulp feedstock treatment, pulp must be diluted to a consistency of 1% to 6%. A mixture of water and ethanol may be used in the described process to reduce viscosity and improve the transferability of the pulp. Using the process of EP '257, MFC is said to be produced after at least ten passes, also known as circulations, through the mechanical refiner, although more passes through the refiner are suggested, resulting in high overall energy consumption.
Conventional methods resisted using medium and high consistency refiners due to the concerns of product quality, such as size and breakage versus desired product properties. Additionally, conventional processes require high energy consumption for refining processes to achieve the desired product, developing a lower energy consumption process is desirable.
Additional information for MFC production is disclosed in the following articles: “Enzymatic hydrolysis combined with mechanical shearing and high-pressure homogenization for nanoscale cellulose fibrils and strong gels” by Pääkkö, M., M. Ankerfors, H. Kosonen, A. Nykänen, S. Ahola, M. Österberg, J. Ruokolainen, J. Laine, P. T. Larsson, O. Ikkala, and T. Lindström (2007) published in the Biomacromolecules 8 (6): 1934-1941; Siró, István, and David Plackett. “Micro fibrillated cellulose and new nanocomposite materials: a review.”. Springer Science+Business Media B.V. Cellulose (2010) 17: 459-494. Web. 4 Sep. 2012; “Processing and Properties of Micro fibrillated Cellulose”, by Spence, Kelley Lynn; Diss. North Carolina State University, 2011. Web; “The build-up of polyelectrolyte multilayers of microfibrillated cellulose and cationic polyelectrolytes”, by Wågberg, Lars; Gero Decher, Magnus Norgren, Tom Lindström, Mikael Ankerfors, and Karl Axnäs (2008), Langmuir 24 (3): 784-795.